Field of the Invention
The present invention relates to methods and inspection apparatuses usable, for example, to perform metrology in the manufacture of devices by lithographic techniques. The invention further relates to computer program products for use in such inspection apparatus and to lithographic systems and methods of manufacturing devices using lithographic techniques.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a diffraction “spectrum” from which a property of interest of the target can be determined.
Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. The targets used by such scatterometers are relatively large, e.g., 40 μm by 40 μm gratings, and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In addition to measurement of feature shapes by reconstruction, diffraction-based overlay can be measured using such apparatus, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables overlay measurements on smaller targets. Examples of dark field imaging metrology can be found in international patent applications US20100328655A1 and US2011069292A1 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple gratings can be measured in one image, using a composite grating target. The contents of all these applications are also incorporated herein by reference.
In the known metrology technique, overlay measurement results are obtained by measuring the target twice under certain conditions, while either rotating the target or changing the illumination mode or imaging mode to obtain separately the −1st and the +1st diffraction order intensities. Comparing these intensities for a given grating provides a measurement of asymmetry in the grating, and asymmetry in an overlay grating can be used as an indicator of overlay error. In addition to overlay, other performance parameters of the lithographic process can be measured using targets of the same general form and by the same general procedure as illustrated in FIG. 4. In particular, targets can be designed in which asymmetry of the target depends, for example, on focus error in the lithographic process, or on exposure dose errors.
Because of the reduced size of the individual gratings in a composite grating target, edge effects (fringes) in the dark-field image become significant, and there can be cross-talk between the images of different gratings within the target. To address this issue, US20110027704A mentioned above teaches to select only a central portion of the image of each grating as a ‘region of interest’ (ROI). Only pixel values within the ROI are used to calculate asymmetry and overlay.
As one considers ever smaller targets, however, the size of ROI that can be defined to be free of edge effects reduces to ever smaller numbers of pixels. Consequently the measurements are inherently more noisy, for a given acquisition time. Moreover, any variation in positioning the ROI becomes a significant source of error in the measured asymmetry.
Edge effects can be very intense. Asymmetric edges of the grating will increase the intensity signal error if they are large and intense enough to end up inside the ROI. For small pattern recognition errors in placement of the ROI, only a small part of the high intensity edge might end up inside the ROI. The small presence of edge effects is not a problem because a conventional 1σ or 2σ-filter is successful in removing these pixels. However, for larger pattern recognition errors, the 2σ-filter will work less well and larger errors in the intensity signal will be introduced.
Oscillations in intensity can occur inside the ROI as a result of the Gibbs phenomenon. This may happen for example when a pupil filter of 0.4 NA (numerical aperture) is used in the scatterometer and a limited number of Fourier components are available to form the grating field image. This results in periodic oscillations of the intensity across the grating image. Because of this effect, a slightly different placement of the ROI on top of the periodic oscillations will result in different average intensities across the ROI. The final averaged intensity is thus very sensitive to the placement of the ROI.
Intensity gradients across the grating image may be caused by the effects of defocus in combination with high NA detection. The final effect is that the ROI has to be placed on a slope in intensity. In this case an error in positioning will result in extra errors in average intensity. It may be expected that these intensity gradients will be more severe in future generations of apparatus, where off-axis illumination and off-axis collection of scattered radiation make the signals more sensitive to focusing errors. Also in the case of focusing errors, images of component gratings within a compound target may move in different directions so as to overlay with one another. Existing signal extraction methods will be increasingly challenged.